Pyrotechnic composition for use as advanced ignition means

ABSTRACT

A pyrotechnic composition, in particular for use in safety devices for motor vehicles, has an auto-ignition temperature between 140 and 180 degrees C., and comprises at least 3-nitro-1,2,4-triazol-5-one (NTO) in a proportion of 20 to 60% by weight. The NTO is stabilized by at least one of the following measures: (a) the NTO has an average grain size of 200 to 600 □m; and (b) the composition comprises an oil in an effective quantity for stabilizing the NTO. The pyrotechnic composition is adapted for use in a safety device, in particular in a safety device in motor vehicles.

TECHNICAL FIELD

The present invention relates to a pyrotechnic composition, in particular for use as auto-ignition means for safety devices in motor vehicles.

BACKGROUND OF THE INVENTION

Safety devices such as gas bag modules, belt tensioners or pneumatic knee restraints nowadays belong to the standard equipment of vehicles. The gas generators of these safety devices contain gas-generating materials such as sodium azide or nitrogen-rich organic compounds, which generally exhibit a very high thermal stability. At high ambient temperatures, such as occuring for example in a vehicle fire, the gas-generating mixtures, however, have a very greatly increased reaction rate. The combustion of the gas-generating mixtures takes place in this case accordingly violently, in unfavourable cases even in the manner of an explosion. The housings of the gas generators are not designed for such a violently occurring reaction and can be destroyed in the process. In addition, the high ambient temperatures, in particular in gas generators with a light metal housing, can lead to a loss of the integrity of the housings. The housings heated by the high temperatures are then so unstable that they already can not withstand the gas pressure developing in a normal combustion of the propellant, and can fragment.

In order to ignite in a controlled manner the gas-generating materials at a high ambient temperature e.g. in the case of a vehicle fire, so-called auto-ignition means are used. Auto-ignition means are exothermally reacting pyrotechnic compositions having an auto-ignition temperature of below approximately 200 degrees C. which is therefore substantially lower than the auto-ignition temperature of the gas-generating materials. The auto-ignition means causes the reaction of the gas-generating mixture being triggered thermally below a critical ambient temperature, even before the stability of the gas generator housings is impaired by the heating and/or the severity of reaction of the gas-generating mixtures has increased too much. In such a case, through the controlled ignition and reaction of the gas-generating mixture, it prevents the destruction of the gas generator housings and the dangers connected therewith for the vehicle occupants or the persons situated in the vicinity of the vehicle.

In the prior art, hitherto stabilized nitrocellulose powders were used as pyrotechnic auto-ignition means. These have an auto-ignition temperature of 180 to 200 degrees C. However, nitrocellulose tends to already disintegrate slowly at low temperatures and therefore does not ensure the capability of functioning as auto-ignition means over the entire life cycle of a motor vehicle. The auto-ignition means on the basis of nitrocellulose therefore generally do not meet the product specifications of the automobile industry currently in force, according to which pyrotechnic compositions must be able to withstand a hot storage over 400 hours at 107 degrees C. whilst maintaining the full capability of functioning.

From EP 0 914 305 auto-ignition compositions for gas-generating mixtures are known, designated as thermal fuses, the auto-ignition temperatures of which range from 150 to 185 degrees C. These pyrotechnic compositions contain for example 3-nitro-1,2,4-triazol-5-one (NTO) and an oxidizing agent, such as for example sodium nitrate. A commercially available composition consists of guanidine nitrate, NTO and sodium nitrate. This composition, with a hot storage of 408 hours at 110 degrees C., shows a weight loss of up to 2% and therefore fulfils the product specifications required for a sufficient reliability of function.

U.S. Pat. No. 5,866,842 A discloses auto-ignition compositions which comprise an oxidizing agent melting at low temperatures, and a fuel. The auto-ignition temperatures of these auto-ignition mixtures range from 130 to 150 degrees C. The fuel forms approximately 10 to 50% by weight of the auto-ignition means and can consist, for example, of NTO.

The compacts produced from the known auto-ignition compositions are, however, unstable in particular in combination with moisture, and disintegrate when they are brought into direct contact with the gas-generating materials. Through this disintegration of the compacts, the function as auto-ignition means is no longer guaranteed. Therefore, the auto-ignition means in the gas generator are usually arranged separated from the gas-generating propellants, in which, however, further measures have to be taken in order to ensure a reliable over-igniting of the activated auto-ignition means to the gas-generating materials.

The present invention is therefore based on the problem of providing a favourably-priced auto-ignition means, able to be handled reliably, with a high stability under storage and with an improved compatibility with respect to other gas-generating materials.

SUMMARY OF THE INVENTION

According to the present invention, a pyrotechnic composition, in particular for use in safety devices for motor vehicles has an auto-ignition temperature of between 140 and 180 degrees C., and comprises at least 3-nitro-1,2,4-triazol-5-one (NTO) in a proportion of 20 to 60% by weight. The NTO is stabilized by at least one of the following measures: (a) the NTO has an average grain size of 200 to 600 □m; and (b) the composition comprises an oil in an effective quantity for reducing a reactivity of the NTO.

In a further embodiment, the invention provides for a method of using the above pyrotechnic composition as an auto-ignition means in a safety device, in particular in a safety device in motor vehicles.

With the invention, the characteristics of NTO known to be favourable as a component of pyrotechnic compositions, such as its reliability in handling, high energy density and capability of ignition, can be maintained. In addition, through the use of the coarse grain NTO or NTO stabilized by the addition of oil, in accordance with the invention, a reactivity of the NTO is reduced which means that the long term stability of the auto-ignition composition with high temperature storage can be considerably improved compared with the conventional commercial mixtures containing NTO. The decomposition reactions, which at hot storage of the commercially available mixtures after 408 h at 110 degrees C. lead to weight reductions of up to 2%, can be prevented or at least distinctly slowed down using the stabilized NTO according to the invention. As the reactivity of the NTO is reduced by the stabilization, in addition, the chemical compatibility with other propellants is improved. It is therefore possible to use the auto-ignition means according to the invention also in direct contact with these propellants and thereby to ensure a reliable over-igniting in the case of activation through increased ambient temperatures.

In the auto-ignition means according to the invention, the oil is present preferably in an amount of additively 0.1 to 3% by weight, i.e. in relation to the overall weight of the remaining components of the composition, such as NTO, further fuels and oxidizers. Basically, all oils can be used which are sufficiently stable with regard to temperature and are compatible with the components of the auto-ignition composition. The oil can be selected from the group consisting of natural and synthetic mineral oils, plant and animal oils and also essential oils and mixtures thereof. Preferably, the oil is a mineral oil, and more preferably a functionalized or non-functionalized silicone oil. These oils are easily available and favourably priced.

Examples of natural and synthetic mineral oils are oils on the basis of long-chained, branched and unbranched hydrocarbons and halogenated hydrocarbons and also oils from the group of aliphatic and aromatic polyethers, polyalkylene glycols, and also the ester oils and silicone oils. Examples of suitable plant and animal oils are those from the group of phospholipids, triglycerides and isoprenoids, in particular olive oil, peanut oil, castor oil, sunflower oil, soya oil, linseed oil, cod-liver oil (fish liver oil), fish oil and marine animal oil.

Furthermore, the pyrotechnic composition can additionally comprise a guanidine compound. The guanidine compound is preferably selected from the group consisting of guanidine nitrate, aminoguanidine nitrate, diaminoguanidine nitrate, triaminoguanidine nitrate, nitroguanidine and mixtures thereof. Guanidine compounds are generally very stable chemically, reliable as regards handling and readily available.

According to a preferred embodiment of the invention, the pyrotechnic composition comprises said stabilized NTO in a proportion of 20 to 50% by weight, said guanidine compound in a proportion of 30 to 50% by weight and from 0 to 50% by weight of an inorganic oxidizer selected from the group of alkali metal nitrates and alkaline earth-metal nitrates. The coarse-grained NTO, optionally together with the oil additive, is regarded here as stabilized NTO. Preferably, the pyrotechnic composition consists essentially of the stabilized NTO in a proportion of 30 to 50% by weight, guanidine nitrate in a proportion of 30 to 50% by weight and sodium nitrate in a proportion of 0 to 40% by weight. Furthermore, the pyrotechnic composition can additionally comprise usual processing aids in a proportion of 0 to 2% by weight. Usual processing aids are, in particular, selected from the group of pressing aids, flowing aids, slip additives, lubricants, separating agents and binding agents such as, for example, methyl cellulose, thermoplastic elastomers, starch, clays, talcum, oxamides, graphite or molybdenum sulphide.

In a further embodiment, the invention is a gas generator for use in a safety device, in particular in a vehicle occupant restraint system comprising a pyrotechnic propellant, an igniter for activating the pyrotechnic propellant, and an auto-ignition means associated with at least one of the pyrotechnic propellant and the igniter, the auto-ignition means consisting essentially of a pyrotechnic composition having an auto-ignition temperature of between about 140 degrees C. and 180 degrees C., wherein the pyrotechnic composition comprises an NTO, stabilized according to the invention. Preferably, the pyrotechnic propellant and the auto-ignition composition are present with each other in a mixture which is stable with respect to storage and in particular is homogeneous.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION EXAMPLES 1 TO 7 AND COMPARATIVE EXAMPLE 8

For producing a pyrotechnic composition suitable as an auto-ignition means, 40 parts by weight guanidine nitrate, 40 parts by weight NTO and 20 parts by weight sodium nitrate were ground, mixed with each other and compacted to form tablets.

In Examples 1 and 3, the average grain size of the NTO was set to 400 □m. The compositions according to Examples 2 to 7 contained the NTO stabilized by the addition of an oil in a proportion of additively 0.5% by weight in relation to the overall weight of NTO, guanidine nitrate and sodium nitrate. Comparative Example 8 corresponds to a commercially available product. The compacts which were thus obtained were subjected to a hot storage over 408 hours at 110 degrees C. and then the weight loss was determined. The results are shown in the following table. Average Grain Size Weight loss after Example No. of NTO/μm Oil Additive Hot storage 1. 400 — 0.97% 2. <10 AK 10 0.25% 3. 400 AK 10 0.14% 4. <10 AK 100 0.19% 5. <10 IM 47 0.89% 6. <10 SAE 15W40 0.03% 7. <10 SAE 5W40 0.00% 8. <10 — 2.15% AK 10; AK 100=synthetic silicone oil, trademark of the company Wacker Chemie GmbH; IM 47=functionalized silicone oil; trademark of the company Wacker Chemie GmbH; SAE 15W40, SAE 5W40=motor oils of the corresponding viscosity class

Examples 1 to 7 according to the invention all show an improved high temperature stability compared with Comparative Example 8. The reactions which in the known composition according to Comparative Example 8 after a hot storage for 408 hours at 110 degrees C. lead to weight reductions of over 2% can already be prevented or at least distinctly slowed down by the use of a stabilized NTO with an average grain size of approximately 400 □m. The weight loss after the hot storage lies at <1% and therefore points to a distinctly longer storability whilst maintaining the full capability of functioning of the composition as auto-ignition means. Also, through the addition of silicone oils or motor oils, the storability can be considerably improved. The measured weight loss lies here likewise below 1%. A combination of oil addition and the use of coarse-grained NTO results in an additional improvement in accordance with Example 3.

Finally, the composition according to the invention, in accordance with Examples 1 to 7, shows a considerably improved compatibility with otherwise incompatible gas-generating compositions. With a storage of the compacts produced according to Examples 1 to 7, in direct contact with a propellant on the basis of guanidine nitrate and copper compounds at 110 degrees C. for 400 hours, no disintegration of the compacts was observed. 

1. A pyrotechnic composition having an auto-ignition temperature between about 140 and 180 degrees C., said pyrotechnic composition comprising 3-nitro-1,2,4-triazol-5-on (NTO) in a proportion of 20 to 60% by weight, wherein said NTO is stabilized, said stabilized NTO comprising at least one of: (a) said NTO having an average grain size of between about 200 to 600 □m; and (b) an oil admixed to said composition in an effective quantity for reducing a reactivity of said NTO.
 2. The pyrotechnic composition according to claim 1, wherein said stabilized NTO comprises an oil admixed to said composition, said composition comprising said oil in a proportion of additively 0.1 to 3% by weight.
 3. The pyrotechnic composition according to claim 1, wherein said oil is selected from the group consisting of natural and synthetic mineral oils, plant and animal oils and also essential oils and mixtures thereof.
 4. The pyrotechnic composition according to claim 3, wherein said mineral oil is one of a functionalized and non-functionalized silicone oil.
 5. The pyrotechnic composition according to claim 1, wherein said composition additionally comprises a guanidine compound.
 6. The pyrotechnic composition according to claim 5, wherein said guanidine compound is selected from the group consisting of guanidine nitrate, aminoguanidine nitrate, diaminoguanidine nitrate, triaminoguanidine nitrate, nitroguanidine and mixtures thereof.
 7. The pyrotechnic composition according to claim 5, wherein said composition comprises said stabilized NTO in a proportion of 20 to 50% by weight, said guanidine compound in a proportion of 30 to 50% by weight and from 0 to 50% by weight of an inorganic oxidizer selected from the group consisting of alkali metal nitrates and alkaline earth metal nitrates.
 8. The pyrotechnic composition according to claim 7, wherein said composition comprises said stabilized NTO in a proportion of 30 to 50% by weight, guanidine nitrate in a proportion of 30 to 50% by weight and sodium nitrate in a proportion of 0 to 40% by weight.
 9. The pyrotechnic composition of claim 7, wherein said stabilized NTO comprises said oil admixed to said composition in an amount of additively between about 0.1 and 3% by weight.
 10. The pyrotechnic composition of claim 9, wherein said stabilized NTO has an average grain size of between about 200 μm and 600 μm.
 11. The pyrotechnic composition according to claim 1, wherein said composition further comprises conventional processing aids in a proportion of additively 0 to 2% by weight.
 12. A gas generator for use in a safety device, in particular in a vehicle occupant restraint system, comprising a pyrotechnic propellant, an igniter for activating said pyrotechnic propellant, and an auto-ignition means associated with at least one of said pyrotechnic propellant and said igniter, said auto-ignition means consisting of a pyrotechnic composition having an auto-ignition temperature of between about 140 and 180 degrees C., said pyrotechnic composition comprising at least 3-nitro-1,2,4-triazol-5-on (NTO) in a proportion of about 20 to 60% by weight, wherein said NTO is stabilized, said stabilized NTO comprising at least one of: (a) said NTO, having an average grain size of about 200 to 600 □m; and (b) an oil admixed to said pyrotechnic composition in an effective quantity for reducing a reactivity of said NTO.
 13. The gas generator according to claim 12, wherein said pyrotechnic propellant and said auto-ignition means are present in a mixture which is stable with regard to storage.
 14. A method of activating a gas generator of an occupant restraint system in vehicles under vehicle fire conditions, said gas generator comprising a pyrotechnic propellant, an igniter for activating said pyrotechnic propellant and an auto-ignition means associated with at least one of said pyrotechnic propellant or said igniter, said method comprising the step of self-igniting said auto-ignition means at a temperature between about 140 to 180° C. thereby triggering a reaction of said pyrotechnic propellant, wherein said auto-ignition means consists essentially of a pyrotechnic composition comprising at least 3-nitro-1,2,4-triazol-5-on (NTO) in a proportion of 20 to 60% by weight, wherein said NTO is stabilized, said stabilized NTO comprising at least one of: (a) said NTO, having an average grain size of about 200 to 600 □m; and (b) an oil admixed to said pyrotechnic composition in an effective quantity for reducing a reactivity of said NTO. 